LTE (Long Term Evolution) is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth. In order to do that, LTE uses Orthogonal Frequency-Division Multiple Access (OFDMA) which is a proven access technique, based on Orthogonal Frequency-Division Multiplexing (OFDM). Other wireless standards like WiFi (IEEE 802.11) or WiMAX (IEEE 802.16) also employ OFDM techniques.
The use of OFDM techniques allow efficient user and data multiplexing in the frequency domain and have many other advantages (for example its ability to resolve the frequency components of the received signal). However, OFDM is highly sensitive to frequency misalignments as well as to Doppler impairments caused by user mobility. Compensation of impairments caused by user speed is of increased importance in wireless cellular systems, particularly in systems employing high carrier frequencies for which Doppler impairments can be very significant. The impact of mobility of the user and/or the environment linearly increases with the carrier frequency. In addition, mobile devices are usually equipped with omni-directional antennas (or present very limited beamforming capabilities), and chances are high that signals are received over a relatively wide angular region. This fact, together with the existence of multipath, transforms Doppler shifts into Doppler spreads which are much more difficult to compensate at the receive side.
Moreover, massive Multiple Input Multiple Output (Massive MIMO) techniques are of increased interest in order to enhance the spectral efficiency per unit area. Massive MIMO (also known as Large-Scale Antenna Systems, Very Large MIMO, Hyper MIMO, Full-Dimension MIMO . . . ) tries to spatially multiplex several users in the same time-frequency resources, thanks to the extra degrees of freedom provided by the high number of antennas at the base station, by employing linear precoding techniques. Extra antennas help by focusing energy into ever-smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include the extensive use of inexpensive low-power components, reduced latency, simplification of the media access control (MAC) layer, and robustness to intentional jamming. There is currently no definition of how many antennas a system must have to be considered Massive MIMO, but a system with greater than 64 antennas is generally considered a Massive MIMO system.
However, massive MIMO systems and traditional cellular systems can greatly suffer from user mobility if OFDM is employed. Doppler impairments give rise to inter-carrier interference and channel estimation impairments, which can be compensated by prior art techniques only up to a certain user speed determined by the actual OFDM frame structure and numerology.
The usual approach when coping with Doppler impairments in OFDM is to estimate the time-domain and frequency-domain channel variations at the receiver side by means of in-band pilots, conveniently interspersed with data subcarriers. Pure Doppler shifts are much easier to compensate than Doppler spreads (due to multipath), the latter demanding adaptive equalization techniques that are upper-limited by the rate of variation of the channel. In addition, equalization is only effective up to a certain user speed above which the channel is no longer constant along the duration of an OFDM symbol.
The subcarrier width can be increased when trying to cope with systems with large Doppler impairments, but this leads to shorter OFDM symbol durations. This is typically interesting when the system bandwidth is large (e.g. as foreseen in millimeter-wave bands, with up to several GHz potentially available for cellular use). If this is not the case, increasing the subcarrier width is not a valid option.
More adequate and effective solutions to compensate Doppler impairments are therefore highly desirable in order to overcome (or at least minimize) the impact of mobility in wireless OFDM cellular networks.